1. Field of the Invention
This invention relates to a pneumatic radial tire having considerably improved ride comfortability, uniformity and side appearance.
2. Disclosure of the Related Art
Heretofore, organic fiber cords such as rayon fiber cords and nylon fiber cords have favorably been used in a carcass of a pneumatic radial tire for passenger cars.
In case of using the rayon fiber cord as a reinforcement of the pneumatic tire, the strength is low as compared with that of the nylon fiber cord, so that if it is intended to manufacture tires based on tire design for safe strength, the amount of the rayon fiber cords used increases to largely increase tire weight and hence the tire performances such as rolling resistance and the like are undesirably degraded. Furthermore, since a raw material for the rayon fiber cord is wood pulp, the use of the rayon fiber cord becomes limited from a viewpoint of problems on resource protection, bad odor produced in the production step, waste treatment and the like.
On the other hand, the nylon fiber cords have a high strength and an excellent fatigue resistance, so that they are fairly used as a reinforcing cord for the tire. However, the satisfactory results are not always obtained in pneumatic radial tires for passenger cars. Because, the nylon fiber cords are low in the modulus of elasticity and large in the creep property, so that when these cords are used as a reinforcement for the tire, the dimensional stability of the tire are poor and the flat spot is undesirably caused.
Lately, polyester fiber cords having a high strength and an excellent dimensional stability are used as a reinforcement for pneumatic radial tires in passenger cars instead of the above rayon and nylon fiber cords. There are proposed many techniques for improving strength, elasticity, shrinkage, heat resistance or fatigue resistance of the polyester fiber cord.
Recently, the tendency for high-grade cars, high user needs and high quality has sharply increased, and it is particularly required to improve a ride comfortability of high-graded cars, reduce an amplitude of vibration or deflection generated through tire nonuniformity (breakage of true circle or the like) and reduce sidewall undulation for a more attractive tire appearance. That is, the above three requirements should be satisfied simultaneously. In case of using the conventionally known polyester fiber cords, however, it is possible to satisfy one or two of the three requirements, but it is impossible to simultaneously satisfy the three requirements.